There are many applications in which it is desirable to have a bandpass filter with high linearity such that a minimal amount of spurious are generated when subjected to an input spectrum with multiple tones. In some applications, it is also desirable for a bandpass filter to be tunable in both center frequency and bandwidth.
Bandpass filters are known in the art. U.S. Pat. No. 3,110,004 relates to a prior tunable bandpass filter, FIG. 1, which includes a multiple tuned resonator filter with each tuned section including a variable reactance device, i.e., a single varactor. The filter also includes a reactive coupling network with its own variable reactance device, and a variable reactance divider at the input of the first resonator which also incorporates a variable reactance device. One disadvantage of the filter in FIG. 1 is that its linearity is less than desirable.
U.S. Pat. No. 5,376,907 relates to another filter, shown in FIG. 2, in which the tuning voltage applied to the resonator varactors is identical to the voltage applied to the coupling varactors. As such, this filter cannot provide independent control of the frequency and the bandwidth.